(Conclusion) Patent infringement may be found under the doctrine of equivalents with the requirements (1) – (5).
Patent infringement may be found, even if the literal difference is found between the claim of the patented invention and the subject product, etc., under the the doctrine of equivalets, if (1) the different part is not the essential part of the patented invention, (2) an identical function and effect can be obtained and the purpose of the patented invention can be achieved by the different part of the subject product, etc., (3) a person skilled in the art could easily come up with the replacement of the different parts at the time of reproduction of the subject product, etc., (4) the subject product, etc. is not identical with the prior art, and could not be easily conceived by a person skilled in the art thereupon, at the time of the patent application, and (5) there are no special circumstances such as prosecution history estoppel.
<Writer: Kei IIDA (Attorney-at-law and Patent Attorney)>
https://www.ip.courts.go.jp/app/files/hanrei_en/640/001640.pdf